Once that has been completed you will be able to edit any of your existing products and assign a specific Mailchimp list to be updated when they are purchased, by clicking the green "Mailchimp Integration" button located in the new "Built In Integrations" section.

(If you are creating a brand new product please submit all of your other settings and then edit it to see the Mailchimp button)

This option will only be available if you've already entered your Mailchimp API and a default/fallback mailing list in your account preferences.

The product list will be updated whenever that product is ordered, instead of the list set within your preferences screen. The list ID given in your preferences screen will only be updated if the order contains products that haven't been assigned to a different list.